1. Field of the Invention
The present invention relates to a wireless LAN (Local Area Network) communication system, and in particular to communication terminals and a relay apparatus that relays communication between the communication terminals in the communication system.
2. Related Art
In recent years, wireless LANs have come to include a communication mode called “infrastructure mode” in which two or more communication terminals perform communication with each other via a specified relay apparatus (hereinafter, called an access point) that controls the network.
IEEE (Institute of Electrical and Electronic Engineers) 802.11 is a typical wireless LAN standard. The IEEE 802.11 standard is described in the following document:
“International Standard ISO/IEC 8802-11: 1999 (E) ANSI/IEEE Std 802.11, 1999 Edition, Part 11: Wireless LAN Media Access Control (MAC) and Physical Layer (PHY) Specifications”
In the IEEE 802.11 standard, when performing communication in the infrastructure mode for example, the access point transmits a beacon signal to each of the communication terminals on the network. The beacon signal includes an SSID (Service Set Identifier) which is an identifier of the access point, management information such as information regarding communication speeds supported by the access point, as well as information indicating whether data addressed to the terminals is stored in the access point.
Receiving the beacon signal enables the communication terminals to be prepared for the transmission/reception of data at any time.
Also, the communication terminals maybe set to a “power save mode” for suppressing consumption of battery power in the terminals.
A communication terminal set to the power save mode operates a reception unit therein only when receiving the beacon signal from the access point, and does not allow the reception unit to operate at any other time, thereby reducing power consumption while still reliably receiving the beacon signal to prepare for transmission/reception. For this reason, if reception of the beacon signal fails, the power save mode is ended, and the communication terminal must standby for reception at all times in order to ensure the reliability of data reception.
However, in the aforementioned wireless LAN, the access point may not be able to appropriately transmit the beacon signal to each of the communication terminals.
Specifically, in the aforementioned wireless LAN, the communication terminals and the access point perform multiple access in accordance with the CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance) method.
For this reason, if data is being exchanged on the network when the access point intends to transmit the beacon signal, a packet collision is predicted, and the beacon signal therefore cannot be transmitted. Also, if the access point receives other data directly before transmitting the beacon signal, there is the possibility that the beacon signal will not be transmitted since data transmission is ceased for a predetermined interval after the reception of data.
In particular, if reception of the beacon signal fails even once while the communication terminal is set to the power save mode, the power save mode is forcedly ended, which is a hindrance to power saving.
The following is a more detailed description with reference to FIG. 21.
In a wireless LAN composed of, for example, an access point (AP) and communication terminals (STA 1 and STA 2) as shown in FIG. 21, the AP transmits the beacon signal at a predetermined time interval (TBTT). However, after the AP has transmitted a beacon signal 2000, a collision occurs between the next beacon signal 2001 and data 2002 since the STA 2 transmits the data 2002 at the same time as the AP transmits the beacon signal 2001, and therefore transmission of the beacon signal 2001 fails. Consequently, the STA 1 that was set to the power save mode does not receive the beacon signal, and must therefore end the power save mode and stay awake.
An aim of the present invention is therefore to provide technology in which an access point can more reliably transmit a beacon signal to communication terminals in a wireless LAN.